Generally, a magnesium alloy has been widely used as a material capable of making parts of a vehicle light, so that the fuel consumption rate is enhanced. The magnesium alloy part adapted to a vehicle is generally fabricated in a die casting process. The die casting process method is directed to fabricating a casting that is the same as the mold by injecting a melted metal into a steel mold that has been precisely fabricated to a casting shape. When fabricating a magnesium part, however, it is impossible to control a casting defect such as a pore, etc. due to a characteristic of a die casting process. Therefore, strength enhancement via heat treatment of castings after die casting is impossible. In addition, an actual shape molding is not performed in the conventional process, so that an expensive machining process is also performed.
As a new forming technique, a thixoforming process has been widely used wherein it is combined with a hot working process including a casting process, an extrusion process and a compression process. The alloys are heated to a solid/liquid coexistence region and are processed by an isothermal holding process during a certain time period. Therefore, the primary solid phases, each having a spherical shape, are uniformly provided in a liquid phase thereby fabricating a slurry. Thereafter, a forming process is performed for fabricating a product.
As a method for fabricating a magnesium alloy using a thixoforming process, according to the Japanese patent No. Hei 8-74015, a billet is fabricated in such a manner that a melt is cooled and hardened at a cooling speed of 1° C./second in a temperature region in which a heating temperature does not exceed 30° C. in a liquidus line in connection with a mold co-heating temperature. The alloy is maintained for 60 minutes at a temperature 0.5° C. higher than a temperature of a solidus line. According to the Japanese patent No. 2001-316753, in order to enhance strength, a magnesium alloy fabrication method is provided based on a thixoforming process in a state that a solid phase is less than 50%. In addition, according to the Japanese patent No. 2003-183794, a magnesium billet is heated in a temperature ranging from 400° C. to 500° C. and is extruded at a container temperature from 380° C. to 440° C., and at an outlet temperature of an extrusions from 400° C. to 480° C. A thin and wider plate material is fabricated with an extrusion ratio from 130 to 670.
In an example of a part for a vehicle using a magnesium thixoforming process, there is a piston for an internal combustion engine disclosed in the Japanese patent laid-open No. 2000-186616. However, the casting material billet used in the thixoforming process requires expensive equipment for agitating the melted mixture during a process in which the billet is fabricated through a cooling process after the casting process is performed. Therefore, the materials for thixoforming process are expensive as compared to the materials for common casting process.
In the product fabricated through the thixoforming process, the size of the primary solid phase largely affects the characteristics of the product. Namely, the agglomeration and coarsening of the primary solid phase disadvantageously affect the mechanical properties of the products during an isothermal holding process in a solid liquid coexistence region after the alloy casting material billet is heated in the thixoforming process. However, when a magnesium alloy is adapted to the thixoforming process, there are not any methods for directly using the common casting material as a billet and for preventing a coursing phenomenon of the primary solid phase.